answered:
100% hydrated. A fully formed wing, very realistic and well made.
75% hydrated. Still fully formed, realistic with some misshapen places at times.
50% hydrated. Malformed, skin begins to dry and magical shape dwindles. Still able to hold misshapen, loose, thin form.
25% hydrated. Wing begins to deteriorate. Only magical form left is the main line. Breaks away like dead leaves, very thin.
0% hydrated. No skin left of the wing, magic shape recedes into the body. Only sign of what once was a wing may be slight lumps of loose skin where they used to be. At this point the kelpie would be running out of magic, and starting to die of dehydration. A glass of water and a dip will restore the kelpies magic and morphing powers.